In the manufacture of patterns, such as electric conductors, on boards, for example copper laminates intended for the manufacture of circuit boards, use is often made of etching to remove portions of an electrically conductive layer.
The board usually has a sheet of a nonconductive material, which sheet is provided with-a copper layer. on one or on both flat sides. Usually a photosensitive coating is applied on the copper layer. A photographic process is used to transfer a desired pattern from a master or a template to the coating, after which the portions of the coating surrounding the pattern are removed. In the etching operation, the copper layer is etched away where the coating has been removed, whereas the copper layer remains in the portions of the copper layer which are still covered by the coating. Etching is often carried out as electrochemical etching, in which a voltage is applied between a cathode and the board, which constitutes an anode, both the anode and the cathode being immersed in an electrically conductive electrolyte. This method of etching is disclosed, for example, in EP 0 889 680 and WO 98/10121. One problem of electrochemical etching is that it is necessary for the portions of the copper layer which are to be etched away to be in continuous electrical contact with the voltage source. If, during etching, a portion of the copper layer loses this electrical contact, this portion will form an “islet” which cannot be etched away. Another problem is that the copper in the copper layer which is oxidised and converted into copper ions will be reduced on the cathode and form metallic copper anew. Since this precipitation is not uniformly distributed over the surface of the cathode, the distance between the cathode and the board will vary over the flat side of the board. Owing to this, also the etching effect will vary over the flat side of the board, implying that some portions of the board are excessively etched and some portions are not sufficiently. etched. To obviate these problems, the distance between the board and the cathode has to be so long that a change of this distance, due to precipitation of copper, will be insignificant in relation to its magnitude. If a long distance is provided between the cathode and the board, it is necessary to apply a high voltage between the board and the cathode to obtain the desired current density and thus a sufficient etching rate. However, if a high voltage is applied, the electric conductors do not obtain the desired dimensions. This in turn prevents the electric conductors on the board from obtaining the intended conductive and resistive properties, which makes it necessary to reject a great deal of the etched boards.